--- old/src/java.base/share/classes/sun/net/util/IPAddressUtil.java	2019-05-17 12:07:36.931649172 +0100
+++ new/src/java.base/share/classes/sun/net/util/IPAddressUtil.java	2019-05-17 12:07:36.643649182 +0100
@@ -25,6 +25,15 @@
 
 package sun.net.util;
 
+import java.net.Inet6Address;
+import java.net.InetAddress;
+import java.net.NetworkInterface;
+import java.net.SocketException;
+import java.security.AccessController;
+import java.security.PrivilegedAction;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
 public class IPAddressUtil {
     private static final int INADDR4SZ = 4;
     private static final int INADDR16SZ = 16;
@@ -287,4 +296,57 @@
         }
         return false;
     }
+
+    /** Returns a scopeId suitable for binding, or -1 if none */
+    public static int bindingScopeId(InetAddress address) {
+        if (!(address instanceof Inet6Address))
+            return -1;
+
+        Inet6Address ia6 = (Inet6Address)address;
+        if (ia6.getScopeId() != 0) {  // explicit scopeId
+            return ia6.getScopeId();
+        }
+
+        int scopeId = -1;
+        if (ia6.isLinkLocalAddress()) {
+            scopeId = IPAddressUtil.scopeFromLocalAddress(ia6);
+        }
+
+        //TODO: if macOS default interface stuff
+
+        return scopeId;
+    }
+
+    private static Map<Inet6Address,Integer> localAddressScopeIdMap = new ConcurrentHashMap<>();
+
+    private static int scopeFromLocalAddress(Inet6Address addr) {
+        assert addr.isLinkLocalAddress() :  addr;
+
+        int scopeId = privilegedLookupScopeId(addr);
+        if (scopeId == -1)
+            return -1;
+        int cur = localAddressScopeIdMap.putIfAbsent(addr, scopeId);
+        assert cur == scopeId;
+        return scopeId;
+    }
+
+    private static int privilegedLookupScopeId(Inet6Address ia6) {
+        PrivilegedAction<Integer> pa = () -> lookupScopeID(ia6);
+        return AccessController.doPrivileged(pa);
+    }
+
+    private static int lookupScopeID(Inet6Address ia6) {
+        try {
+            return NetworkInterface.networkInterfaces()
+                    .filter(nif -> nif.inetAddresses()
+                                      .filter(addr -> addr.equals(ia6))
+                                      .findFirst().isPresent())
+                    .findFirst()
+                    .map(NetworkInterface::getIndex)
+                    .orElse(-1);
+        } catch (SocketException ignore) {
+            // ignore, handle as not found
+        }
+        return -1;
+    }
 }